Special purpose container handling cranes are used in modern cargo handling facilities to load and off-load ships, and the speed with which ships may be serviced by such cranes is a key determining factor in the overall efficiency of a port. Current gantry cranes use a head block to grapple a container by way of a spreader. The head block is suspended from a rail mounted gantry trolley through an arrangement of reeving cables used to raise and lower the head block.
The cycle times of gantry cranes are limited by two main factors. Firstly, containers may not always be aligned in such a way as to allow easy positioning of the spreader on top of a container that is to be lifted. Thus, the speed with which the spreader may accurately be positioned by the crane is of considerable importance. Typically, this positioning may take up to 50% of the duty cycle of the crane, the remainder being taken up by travel between ship and shore. Secondly, the head block and load suspended from the reeving, at heights sometimes up to 50 m, have the tendency to sway or swing during motion of the crane and during hoisting operations. These factors can reduce so-called box rates and, hence, port efficiency by about 50%. Considering that a typical container ship may require up to 1,000 container movements, potential benefits of improving crane efficiency are substantial.
Reducing the sway of the crane hoist and making the reeving more controllable will result in reduced positioning times and improved ship service times. A reduction in positioning time of the order of 10-20% would have a substantial impact on the cargo handling business.
Shipyard and quay-side cranes that currently are employed are stable only in the vertical, z-direction. Loads carried by such cranes may be caused to rotate and sway under lateral forces.
Industrial practice currently employed to improve the operating efficiency of such cranes relies on using the skill of the crane driver to avoid load sway and on the use of complex anti-sway systems. Such systems usually employ complex reeving arrangements and active control systems for hoist motors.
Two such active sway suppression systems are disclosed in U.S. Pat. No. 2,916,162 and U.S. Pat. No. 4,350,254. The former system tends to suppress the pendulum motion in the horizontal direction but fails to suppress any pitch, roll or yaw of the load. The latter system employs additional wires and winches.
A number of studies have been conducted toward achieving more effective reeving arrangements.
One such study has been published by N. G. Dagalakis et al in an article entitled "Stiffness Study of a Parallel Link Robot Crane for Ship Building Applications", Journal of Offshore Mechanics and Arctic Engineering, August 1989, Volume lll, pages 183-193. A further study has been published by James Albus et al under the title "The NIST robot crane", Journal of Robotic Systems, 10(5), 1993,pages 709-724.
Both of these studies disclose cranes having improved hoist and reeving arrangements incorporating the concept of an inverted Stewart Platform of the type commonly used in aircraft simulators. In the cranes by Albus and Dagalakis, the parallel links between the base and the supported load of a Stewart platform are replaced by the reeving cables of the crane, and winches are used as the actuators.
The hoist and reeving designs proposed by Dagalakis and Albus involve the provision of connection points for the reeving on a lower load platform at the vertices of an equilateral triangle. In a similar fashion, connection points for the reeving on the crane trolley are arranged at the vertices of an equilateral triangle. Six reeving cables run from the trolley to the lower load platform, two being connected at each vertex of each triangle. As viewed in plan from above, the upper and lower triangles are rotated through 180.degree. about a vertical axis with respect to one another, so that the vertices of the lower triangle are positioned to align with the mid points of the sides of the upper triangle.
It can be shown that the reeving arrangement disclosed by Dagalakis and Albus will be capable of supporting a load while maintaining tension in all cables, so as to provide stable positional control of the load, only when the centre of mass of the load is contained within the geometric triangle whose apexes are fixed by the location of the sheaves on the lower load platform. It can also be shown that in order to provide stable positional control, the radius of the circle circumscribing the triangular sheave arrangement on the lower load platform will be approximately 1.2 m for a load platform or head block which is arranged to support a 2.4 m wide .times.3.0 m high .times.12.0 m long standard container. Similarly, it can be further shown that the radius of the circle circumscribing the triangular sheave arrangement on the trolley will then be approximately 2.4 m.
These geometrical constraints in turn establish an allowable eccentricity of the centre of mass of a load contained in a standard container suspended by such reeving arrangement of .+-.0.6 m in the lateral direction (along the gantry on which the trolley is moveable) and .+-.0.7 m in the longitudinal direction (direction perpendicular to the gantry). These figures translate to an allowable centre of mass eccentricity of 24% and 6% in the lateral direction and longitudinal direction, respectively. While the allowable lateral eccentricity of 24% is greater than the typical industrial standard specification of 10%, the 6% eccentricity in longitudinal direction does not meet the industry standards.
The present invention seeks to minimise the above mentioned difficulties.